The field of invention is character recognition systems and more particularly magnetic ink character recognition systems for reading documents wherein the characters are printed with magnetic ink.
The prior art devices utilized for reading documents printed with magnetic ink used, for the most part, a single read head for reading the magnetically encoded character. Most typically utilized, in more recent types of equipment, is the alternating current type of read head. However, there are a large number of older systems that use a direct current type of read head. In practice it has been discovered that each of these types of read heads have its own particular advantages and disadvantages. For example the direct current read heads have the advantage of being able to read more deeply into the document so that when the ink encoded character is covered by cellophane tape or similar substances the direct current head is still able to read the character. In addition, direct current read heads are quite effective in reading documents that have been creased or folded. On the other hand, direct current read heads are not always able to correctly interpret documents that have correction labels affixed over the original characters since the deep reading characteristics of the head tend to cause it to also react to the character inscribed underneath thereby causing misreads. In addition the direct current read heads are significantly more likely to misread, or substitute characters, due to misprinted or skewed characters on the document than are alternating current read heads.
On the other hand, alternating current read heads have the disadvantage of not being able to read documents that are creased with the same facility as direct current heads nor, due to the fact that they read a shallower field, are they able to read characters that are covered by cellophane tape or the like. The alternating current read heads do, however, have the advantage of having significantly greater overall accuracy in reading characters and thus have a substantially lower reject rate than the direct current read heads. It is this improved rejection rate that has led to the use of alternating current read heads on most of the newer magnetic character reading devices.
In developing the invention, it was discovered that by utilizing both an alternating current read head and a direct current read head in the same system that a significantly lower reject rate could be achieved for documents passing through the reading device. One of the principle advantages of the two read head system consists of being able to read a character with one head when the other read head is unable to recognize that particular character. Also in the event that the character read by each of the heads does not agree, it is possible to either select the character identified by the head with the greatest overall accuracy, generally the alternating current read head, and to use that as the identified character or to insert a reject character in place of the identified characters. By utilizing these two different reading techniques, in a magnetic ink character recognition system it is possible to significantly reduce the rejection rate of documents passing through the system.
In the prior art, there have been incidences of the use of more than one read head for sensing the same character in a magnetic ink character recognition system, specifically U.S. Pat. No. 3,629,822, where each character is read by two heads at the same time in order to get an improved signal identifying that character. But the prior art does not show the use of two separate read heads that utilize two entirely different reading techniques, i.e. alternating current and direct current, to make two separate readings of each character and to logically compare the results.